The present invention generally relates to brake actuating elements and more particularly relates to an actuating unit for an electromechanically actuated disc brake for automotive vehicles.
EP 0 402 421 B1 discloses an electromechanic actuating unit of this type. The electric motor drives the reducing gear by way of the freewheel mechanism in the prior-art actuating unit. The freewheel mechanism which permits a frictional torque transmission prevents a rotational movement of the electric motor in the direction opposite to its actuating direction or in the release direction of the brake when reaction forces occur and, thus, acts as a parking brake.
What is disadvantageous in the prior-art actuating unit is, especially, the functional principle of the freewheel mechanism employed, the clamping effect of which will slowly yield in a high-frequent excitation or under vibration stress due to a micro-sliding action in the loading direction so that the retaining force required cannot be ensured. This means that a freewheel mechanism of this type is inappropriate for use under safety aspects, such as e.g. in the operation of a parking brake, where it is absolutely imperative to maintain the holding effect.
In view of the above, an object of the present invention is to disclose an electromechanic actuating unit of the type initially referred to wherein an inadvertent release of the parking brake is prevented, thereby increasing its reliability in operation. Another objective is to achieve a space-saving, compact construction of the electromechanic actuating unit.
According to the present invention, this object is achieved in that the freewheel mechanism is configured so that its clamping effect is ensured by a form-locking torque transmission, and that, in its actuated state, it prevents a rotational movement of a bearing in which the rotor of the electric motor is mounted. Preferably, the bearing may be configured as a ball bearing, a needle bearing, or a roller bearing.
In a preferred embodiment, the present invention includes a freewheel mechanism forming a subassembly with the bearing, wherein both the outside and the inside ring of the bearing are extended on one side in such a fashion that they enter into a form-locking engagement with the clamping element of the freewheel mechanism. These measures permit the integration of components in a much more compact manner.
In a preferred aspect of the subject matter of the present invention, the inside ring of the bearing has a profile which permits a form-locking accommodation of the clamping element, and the outside ring has at least one radial recess and a subsequent slope or ramp which, along with the profiling, limits at least one clamping slot in which the clamping element is received.
In another embodiment of the present invention, the clamping element is biassed in the direction of the recess by means of a spring element. Favorably, the spring element is configured as a circlip or a leaf spring.
An actuation of the parking brake of the present invention which is easy to realize and ensures a reliable operation is achieved according to another feature of this invention because the freewheel mechanism is operable by means of an electromagnetic actuating unit.
The electromagnetic actuating unit is preferably comprised of an electromagnet and a tappet which is movable into a force-transmitting engagement with the clamping element, and the electromagnet is designed as a bistable electromagnet.
The clamping element may be configured as a jamming roller or a ball.
In a particularly favorable embodiment of the subject matter of the present invention, the reducing gear is configured as a ball-and-thread drive assembly, the actuating element being the threaded nut of the ball-and-thread drive assembly.
To considerably reduce the necessary drive torque to be produced by the electric motor, the force transmission between the electric motor and the reducing gear is effected by means of a second reducing gear.
In a preferred aspect of the subject matter of the present invention, the electric motor, the (first) reducing gear, and the second reducing gear are designed as at least two independent assemblies so that the electric motor is arranged outside the flux of force of the clamping force and its function cannot be impaired by disturbing influences. This arrangement has an additional benefit because it uncouples the second reducing gear from the (first) reducing gear so that, especially when the second reducing gear is configured as a planetary gear, an equal position of the rotor of the electric motor in relation to the planet wheels and of the planet wheels in relation to the ring gear can be ensured. Designing the second reducing gear as a planetary gear permits considerably reducing the necessary drive torque that is to be generated by the electric motor, with the planetary gear representing an anti-friction gear which does not require any work of deformation and allows achieving a high efficiency with a small mounting space.
It is especially favorable that the electric motor, the first reducing gear, and the second reducing gear is designed as one independent subassembly each. An electromechanic actuating unit of a like design is characterized by high efficiency, extraordinary dynamics of brake actuation, and an extremely compact construction permitting high mass-related brake torques to be transmitted. Besides, subassemblies of a modular design can be constructed and tested separately.
In a favorable aspect of the subject matter of the present invention, the second reducing gear is arranged on the side of the electric motor remote from the brake linings. This measure permits a constructive uncoupling of the second reducing gear from the first reducing gear so that deformation of the second reducing gear is effectively prevented and clearances can be maintained constant within the gear unit.
A higher gear ratio is achieved in another embodiment of the subject matter of the present invention in that the second reducing gear is configured as a planetary gear, preferably with stepped planet wheels. It is, however, also possible to design the second reducing gear as a two-step differential planetary gear. In the last-mentioned gear type, an optimal overall length is achieved because a large sun wheel can be used.
According to another feature of the present invention, uncoupling of the flux of force from the drive unit or the electric motor is ensured because there is provision of a guide member which is supported on a casing that accommodates the ball-and-thread drive assembly and embraces the threaded nut, the threaded spindle being axially supported on the guide member. The axial support of the threaded spindle is carried out by the intermediary of an axial bearing by means of a radial collar. This allows using a bearing with a very small diameter.
It is particularly advantageous that force-measuring elements are provided on the guide member, thereby allowing to effect force measurements at the part that is not moved and is subject to a defined deformation.
An effective protection of the arrangement against contaminants and the ingress of water is achieved due to an elastic seal or sleeve that is interposed between the threaded nut and the guide member.
In another favorable embodiment of the subject matter of the present invention, the sun wheel of the planetary gear is designed on the rotor, while the planet wheels are mounted in a planet cage (which is in a force-transmitting connection with the threaded spindle) and are comprised of a first planet wheel of large diameter that is in engagement with the sun wheel and a second planet wheel of small diameter that is in engagement with a ring gear.
The mounting space is optimized in the above-mentioned design because the ring gear of the planetary gear is formed by an internal toothing in a cover which represents a case of the planetary gear and is mounted on the casing of the electric motor.
In another embodiment of the subject matter of the present invention, the assembly of the actuating unit of the present invention is considerably simplified because the transmission of force between the planet cage and the threaded spindle occurs by means of a form-locking plug coupling.
A low-cost design of the actuating unit of this invention involves that the planet cage is mounted in the cover by means of a radial bearing. A planetary gear of this type is easy to manufacture and allows separate testing.
It is suitable that the form-locking plug coupling is coupled to the planet cage in a torsion-proof, radially yielding and flexible fashion. This measure allows an effective isolation of disturbing influences.
The threaded spindle may preferably have a multi-part design.
In another favorable design of the subject matter of the present invention, the threaded nut at its end remote from the first friction lining includes an axial projection which is movable into abutment with a stop that is provided on the threaded spindle also in an axial direction and acts in a circumferential direction. Jamming or clamping of the first reducing gear is prevented by this provision, in particular in a faulty release action, where the threaded nut is reversed until its stop.
In further favorable embodiments of the present invention, the electric motor may be configured as an electronically commutated electric motor (non-brush d-c motor) energized by a permanent magnet, or a switched reluctance motor (SR=Switch Reluctance motor).
The types of motors mentioned above are especially suited to produce high torques during standstill.
In order to electronically commutate the motor of the actuating unit, it is necessary to provide a position detection system which permits detecting the position of the rotor of the electric motor in relation to the stator and, preferably, includes a Hall sensor or a magnetoresistive element.